I. Field of the Invention
The present invention relates generally to the field of global satellite communications, and more specifically, to re-synchronizing digitally sampled data using a memory device.
II. Description of the Related Art
In recent years, a milestone in the field of telecommunications has been the development of low earth orbit (LEO) satellites for use with wireless communications devices. With LEO satellites, a user with a wireless telecommunications device, such as a handheld or mobile radio-telephone or station, a mounted or fixed mobile radio-telephone, or a paging/messaging-type device, can access another user with little signal path delay.
In one example of a satellite communications system, forty eight LEO satellites are provided in 1414 kilometer LEO, with six of these satellites distributed in each of eight orbital planes. These planes are distributed at 52 degree orbits with respect to the equator, such that each satellite completes an orbit every 114 minutes. This permits full coverage of the earth, with at least two satellites within range of a user located between seventy degree north latitude and seventy degree south latitude at any given time.
In a typical scenario, the user""s analog voice data is digitized, compressed, and modulated by the mobile radio-telephone for transmission to the LEO satellite. This modulation, where the signal is multiplied by a carrier wave for external transmission, can take many different forms. In a code division multiple access (CDMA) spread spectrum modulation technique, a sinusoidal carrier wave is quadraphase (four phase) modulated by a pair of pseudonoise (PN) sequences. These PN sequences provide the spreading code that is transmitted by a single cell sector, identifying the cell location. In an exemplary system, One PN sequence bi-phase shift key (BPSK) modulates the in-phase (I) channel of the carrier, while another PN sequence bi-phase shift key modulates the quadrature (Q) channel of the carrier. The latter bi-phase shift key is referenced as the QPSK.
Additionally, in the CDMA technique, the signal is covered or channelized with an orthogonal code generally generated by using Walsh functions. This orthogonal code is referred to as a Walsh sequence, and serves to identify the particular mobile radio-telephone for the call duration. The Walsh sequences, which represent different user code channels, are preferably orthogonal to one another in order to reduce mutual interferences, and permit better link performances and higher capacities. An identical Walsh sequence is generally applied to the I and Q channels, resulting in bi-phase modulation for the orthogonal codes. Because neighboring cells and sectors have different PN spreading codes, neighboring cells and sectors can reuse Walsh sequences (channel codes). The resulting signal is an intermediate frequency (IF) data stream.
For transmission to the LEO satellites, the IF signal must be converted to the higher radio frequencies (RFs). This is implemented by one or more additional modulations, where the signal is multiplied by higher frequency carrier waves. Here, the signal is said to be xe2x80x9cup convertedxe2x80x9d and the CDMA modulated signal is said to be xe2x80x9cput on top ofxe2x80x9d a higher frequency carrier. Typical frequencies used by LEO satellites include the L band RF, ranging between 1.61 GHz and 1.625 GHz, the S band RF frequency, ranging between 2.485 GHz and 2.5 GHz, and the C band RF frequency, ranging between 3 GHz and 7 GHz.
In a preferred communication system the LEO satellite functions as a xe2x80x9cbent-pipexe2x80x9d receiver. It receives the signal transmitted by the mobile radio-telephone, converts the frequency of the signal, and retransmits the frequency-converted signal to a gateway base station. The gateway functions as an intermediary between one party using a mobile radio-telephone and a second party using another device, which may be another wireless device such as a mobile radio-telephone, or a wired telephone. The gateway can provide communication with both Public Switched Telephone Network (PSTN) telephones and non-PSTN telephones. The LEO satellite operates to transfer signals in both call directions, i.e., to frequency-convert signals arriving from the gateway (from a PSTN or non-PSTN terrestrial telephone) and transmitted to the mobile radio-telephone, and vice versa.
Once the gateway receives the modulated signal, it must reverse the processing performed by the mobile radio-telephone to recapture the original information signal usually analog voice data. These functions are performed by a receiving antenna connected to a transceiver or receiver signal processing system referred to as a receive rack and demodulators using signal processing circuits which may be in the form of application specific integrated circuits (ASICs). These components are part of the gateway transceiver subsystem (GTS) of the gateway.
The receiving antenna system or analog receiver xe2x80x9cdown convertsxe2x80x9d the received RF signal (which is typically in the L, S or C bands) and transmits the signal to the receive rack. The receive rack further down converts the signal to an IF signal, an A/D converter_converts the signal by performing sampling, and transmits the signal to the demodulator ASICs. The demodulator ASICS mix the IF signal with outputs from I channel, Q channel and Walsh code PN generators, in order to retrieve the original analog voice signal.
Unfortunately, there are limitations created by the structure of the gateway. In a typical cellular base station system, which permits direct (i.e., without satellites) communication between mobile radio-telephones and base stations, each base station transceiver subsystem (BTS) typically supports only three sectors. However, in the LEO satellite system, many beams and associated subbeams, on the order of 16 beams per satellite, are supported by a single GTS, where a xe2x80x9cbeamxe2x80x9d is the equivalent of one sector.
The exorbitant volume of information requires that the RF signal received from a LEO satellite be segmented in the receive rack for separate processing. These segmented portions are typically called receive shelves. The outputs from a number of receive shelves must be synchronized before being transmitted to the demodulators using demodulator ASICs for modulation. Synchronization of the outputs is problematic, because there may be delays introduced by accessory components (such as conductors or cables) that are used to convey the fragmented signals to the demodulator ASICs. When data of the same frequency (i.e., isochronous data) is phase-misaligned in this manner, it is commonly referred to as xe2x80x9cskewed.xe2x80x9d
Another source of phase misalignment is the Doppler effect. Information transmitted from a satellite as it approaches a gateway is received at such a bit rate as if it was transmitted at a higher bit-rate because the speed of the satellite and the information transmitted from the satellite are additive. Similarly, information transmitted from a satellite as it moves away from a gateway is received at a lower bit-rate because the speed of the satellite ebbs the rate of transmission. This higher or lower speed of transmission means that the signal received by the gateway is at a higher or lower frequency than expected. This phenomenon is not as readily seen in other forms of communication, whether satellite or terrestrial, because LEO satellites travel at very high velocities relative to the receiving units (either the gateway or the mobile radio-telephone). Accordingly, there is a need to synchronize the received satellite RF signal, regardless of the increased or decreased number of bits resulting from the Doppler effect. There is also a need to ensure that the demodulation process continues reasonably error free, in the event that a synchronization failure occurs, to ensure that a good voice data signal is received by the mobile wireless device.
The present invention is directed to a re-synchronizing phase-independent first-in first-outxe2x80x3 (FIFO) memory. The re-synchronizing phase-independent FIFO memory aligns digital data transmitted between various signal processing elements such as found in the receive shelves and the digital shelves in a gateway transceiver subsystem (GTS) of a low orbit (LEO) satellite communication system.
RF data transmitted from a LEO satellite is transmitted to an antenna, which is connected to circuit elements that down converts the data and transmits the data to the receive distribution shelf of a receive rack. The receive distribution shelf copies the data, and passes each data stream to a number of receive shelves. The receive shelves filter the data to capture a section of the RF spectrum, down convert the data to an IF frequency range and clock the data into the digital shelves. In the digital shelves, demodulator ASICs demodulate the data to retrieve an original signal sent by a mobile radio-telephone user.
To prevent misalignment of the data streams between the receive shelves and the digital shelves, the re-synchronizing phase-independent FIFO memory uses separate and independent input and output signals. Specifically, an input clock (CLK_IN) signal and an input synchronization (EVEN_SEC) signal are used to align data leaving each portion of a receive shelf. An independent output clock (CLK_OUT) signal and an independent output synchronization (SYNC_OUT) signal are used to align the data between the different segmented portions of each receive shelf, as this data goes into the digital shelves. In this manner, phase misalignments (due to disadvantageous Doppler effects or component characteristics) are removed.